MC352 into varying-load speaker?

Kck: Autoformers act as a buffer. If you have an anemic amp and a speaker system that presents a wild and highly reactive impedance as a load, that "buffer" could be a good thing. Otherwise, that "buffer" acts as a sonic veil between the amp and speaker interface, lowering the sound quality. Using an autoformer with a good amplifier would be equivalent to running hundreds of feet of small gauge highly inductive speaker cable. Would you ever consider doing this under normal circumstances??? Sean
>

The autoformer doesn't work like Sean thinks it does.. Mcintosh output transformers are bifilar wound which is a complex technique used to nearly eliminate the inductance associated with the wire. We use it in the lab for special EMI filters (different core setup though). The coupling is just about perfect with a larger bandwidth than the output transistors. Just like tube amps - only better because not all manufactuers have the ability to bifilar wind their trannies.

Aball: Has Mac always used a bifilar design or is that what they've gone to on more recent models? While i saw the test results / bandwidth on one of their more recent models, and it looked pretty impressive for an output transformer coupled model, i don't remember the older models being anywhere near as linear over that wide of a bandspread. As such, either their transformers have changed or the designs of their amps have improved with age.

Either way, the transformer still acts as a buffer. Rather than directly dealing with the input of the speaker and the reflected EMF that it would present, the output stage sees the transformer first with the speaker as a secondary load. As i mentioned, this increases the stability of the amp as it always sees a somewhat consistent load. That "consistent load" is also what gives these amps their "house sound" and electrical characteristics. If you like them and it will do what you want, buy it and enjoy it. Sean
>

Yes, all autoformer McIntoshes use bifilar winding (trifilar on some amps). McIntosh patented it in 1947 for their "unity-coupled" circuit, originally for tube amps of course but then they applied it to solid state in the early 60s and have ever since. I have performance data for a MC2255 and the bandwidth is nearly as wide as today's models.

I have tried Mc amps with autoformers and without, concluding that there is actually little sonic difference between them (when comparing appropriate models). The autoformer mainly acts as a stabilizer, as you said. I currently use a direct-coupled McIntosh amp myself. I like them either way. Arthur

The MC352 is a stereo amp which has two 225 watt amplifiers per channel that are fully balanced through the output Autoformer.The two amps in each channel are floated above chassis ground. The autoformer is a transformer with a coil of wire that ends in a ground point. The two amps feeding the autoformer produce one half of the sine wave each and are combined in the autoformer. The amps crossover point will float to maintain balance and if one of the amps is shorted out the other amp will complete the sine wave at its 225 watts. This design is called Quad balanced by McIntosh and will result in a signal to noise ratio of over 124 dB. This design of course uses common mode rejection.
Like a direct coupled amp the MC352 will double power if say a 2 ohm load is put on the four ohm tap. Heat and current draw will also go up. Unlike a direct coupled amp the MC352 will not halve power as the impedance is increased. With the MC you get all the power you pay for regardless of the speaker used.
The output autoformer allows the amp section to be designed for maximum linearity and efficiency which may result in say an impedance of 2.37 ohms. Since there are not a lot of 2.37 ohm speakers the autoformer is then taped to yield the 2,4, and 8 ohm point. This results in an increase in power over compromised direct coupled designs.
The autoformer has a greater bandwidth and power capability than the amp section which drives it. The MC will put out rated power to 100 KHz and swing high output current. The autoformer is an audio 'transmission' to match the amp to a variety of loads. In no way is a properly designed and built autoformer a 'buffer'.
ron-C